Vapor generating organziation and method



Dec. 5, 1967 F. J. HANZALEK VAPOR GENERATING ORGANIZATION AND METHOD 3 Sheets-Sheet 1 Filed July 12, 1965 B 005 TEE PUMP INVENTOA. FREDE/P/CK a. HANZALEK BY [Zak Dec. 5, 1967 F. J. HANZALEK VAPOR GENERATING ORGANIZATION AND METHOD Filed July 12, 196E v 5 Sheets-Sheet 2 3 lN/T/AL RIF/45147751? FIG 2 INVENTOR.

- mmsp/cx J. HAA/ZALL-k Dec. 5, 1967 F. J. HANZALEK 3,356,074

VAPOR GENERATING ORGANIZATION AND METHOD led July 12, 1965 3 Sheets-Sheet 5 INVENTOR. FREDER/CK (1 fiMA/ZALEK' United States Patent 3,356,074 VAPOR GENERATING ORGANIZATION AND METHOD Frederick J. Hanzalek, Sufiield, Conn, asslgnor to Combustion Engineering, Inc., Windsor, Comm, a corporation of Delaware Filed July 12, 1965, Ser. No. 471,029 7 Claims. (Cl. 122406) ABSTRACT OF THE DISCLOSURE A forced through-flow vapor generator having a flash tank in bypass relation with valve means interposed between heat exchange surfaces in the through-flow system with the flash tank being operative for startup purposes in conventional fashion. After startup and during normal operation of the vapor generator heat is imparted to the flash tank to provide a head of relatively low pressure steam. This heat may be imparted by bleeding hot fluid from the through-flow system or by other heating arrangements. This head of steam is utilized to supply steam for the urbine drive of the feedpump during a hot restart.

This invention relates generally to the art of vapor generation and is particularly concerned with a forced through flow vapor generator and operating method as concerned therewith.

In the operation of forced through flow vapor generators as employed in the power plant industry it is desirable at times to initiate operation of the vapor generator while the generator is in what is termed a hot condition. These vapor generators form part of an electric generating system with the vapor generators supplying vapor to turbo generator units which, in turn, generate electric power. It sometimes happens that the electric load supplied by the turbo generator is lost for a relatively short time. With this loss of load the demand for vapor is, of course, eliminated and the vapor generator must, therefore, be shut down. In re-starting the vapor generator after a relatively short shutdown of this type it is necessary to force the Working medium through the through flow vapor generator and apply heat to the generator. However, the feed pumps employed with these generators are conventionally driven by a vapor turbine which, during normal operation, receives its vapor supply from the vapor generated in the vapor generator. Thus in the absence of special startup pumps there exists a problem in connection with initiating operation of the vapor drive for the feed pump. Furthermore, even with electric motor driven startup pumps a serious problem presents itself to provide a rapid re-startv because of the massiveness of the electric motor drive and the pump. In addition to providing a hot re-start on complete loss of load the operation may be such that the main load is lost for a short time with it being desirable to maintain operation of the turbo generator unit at a reduced load such as of maximum in order to supply suflicient electricity for what is termed the house load, i.e., the load necessary to operate some or all of the electric driven equipment in the power plant. Since during normal operation the vapor driven turbine for the feed pump receives its vapor supply from an extraction point in the main turbine cycle a problem exists as to the continued supply of vapor to this feed pump turbine at this very much reduced load since the vapor at the extraction 3,356,074 Patented Dec. 5, 1967 point would then be at such a low pressure as to be incapable of operating the turbine drive for the feed pump.

With the method and apparatus of the present invention, however, there is available an adequate vapor supply for operating the turbine drive of the feed pump whereby a hot re-start may be effected and a very low load operation maintained if necessary. In accomplishing this objective there is provided a source of vapor that is available at all times and that is maintained in such avialable condition during normal operation of the vapor generator. This source of vapor is stored in the flash tank or separator which normally forms a part of the forced through flow vapor generator and that is utilized in starting the vapor generator from a cold condition.

Accordingly it is an object of this invention to provide an improved method and apparatus relating to the generation of vapor and the production of electric power.

Another object of this invention is to provide such an improved method and apparatus utilizing a once through flow vapor generator whereby simplification of the initiation of operation of the vapor generator is obtained while the vapor generator is in a hot condition.

A still further object of the invention is to provide such an improved method and apparatus wherein a vapor supply is maintained available for a hot re-start of the vapor generator such that the turbine driven main feed pump may be utilized for initiating such hot re-start.

Other and further objects of the invention will become apparent to those skilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises an arrangement, construction and combination of the elements of the inventive organization in such a manner as to attain the results desired, as hereinafter more particularly set forth in the following detaileddescription of an illustrative embodiment, said embodiment being shown by the accompanying drawings wherein:

FIG. 1 is a diagrammatic representation in the nature of a flow diagram disclosing the improved power plant embodying the present invention; I

FIG. 2 is a modification of FIG. 1, disclosing a different arrangement for maintaining a vapor supply in the separator or flash tank during normal operation; and

FIG. 3 is a still further modification showing still another way of maintaining the vapor supply in the separator or flash tank during normal operation.

Referring now to the drawings, wherein like reference characters are used throughout to designate like elements, the illustrative and preferred embodiment of the invention as depicted therein includes a once through flow vapor generator that supplies vapor to a turbine arrangement which, in conventional practice, forms part of a turbo generator unit and drives a generator for the production of electric power. The once through flow vapor generator may be of the supercritical or subcritical pressure type and it is divided into what may be termed a. superheater sec-tion designated generally 10 and within which at least a major portion of the superheat of the working fluid is effected and a vapor generating section designated generally 12. In the supercritical pressure arrangement it is in this latter section that the fluid is heated generally to somewhat above the critical temperature. In the subcritical arrangement it is in this section 12 that the Working fluid may be evaporated to a vapor and the vapor heated somewhat above saturation. The vapor generating section of the through flow system includes the economizer 14, the furnace center wall tubes, the outer furnace wall tubes and the rear pass tubes 16, 18 and 20, respectively.

The through flow is conveyed serially through these various heat exchange sections and on into the superheating section of the through flow circuit which is made up of a number of heat exchange bundles 22 connected in series flow relation as disclosed.

The vapor egressing from the final superheater section through conduit 24 is conveyed to the turbo generator unit 27 entering first turbine stage 26 with this being the high pressure turbine stage and from this high pressure turbine stage the vapor is conveyed to the low pressure turbine stage 28 and then to the condenser 30. Condensate is pumped by the condensate pump 32 from this condenser through the demineralizer 34, low pressure heater 36 and into the deaerator 38. From the deaerator the feedwater is pumped by means of the feedwater booster pump 40, which is an electric motor driven pump, through the intermediate pressure heater 42, the main feedwater pump 44, the high pressure heater 46 and then through the flow control valves 48 and into the through flow circuit of the vapor generator. The electric motor driven feedwater booster pump 40 may be required for proper operation of the main feedwater pump 44 since these high capacity, high head pumps often have what is termed a net positive suction head problem wherefore they require a predetermined positive inlet pressure in order to function in the manner that they are intended and designed. Intermediate the vapor generator section 12 and the supcrheater section of the through flow circuit there are provided valves 62 which control the through flow through these sections. These valves are normally utilized during the startup process employed with the vapor generator. Connected in bypass relation with these valves is the separator or flash tank 64 with this tank being connected with the through flow circuit upstream of valve 62 by means of the conduit 66 within which is disposed the throttle valve 68. The upper region of the flash tank is connected with the through flow circuit downstream of valve 62 by means of conduit 70 and within this latter conduit is disposed the valve 72. This upper region of the flash tank is also connected with the condenser 30 with there being disposed in this connection the SP valve with this valve preventing an over-pressure within the tank 64. This flash tank is normally utilized for a. cold start of the vapor generator and for the procedure of operation with regard to the cold start reference is hereby made and there is hereby incorporated into this application the disclosure and description of my U.S. Patent 3,194,219, issued July 13, 1965.

In the method and organization of the invention the feed pump 44 is driven by the vapor turbine 67. During normal operation of the vapor generator this turbine receives its vapor from an extraction point in the turbine 26 with the vapor being conveyed through line 86 to the turbine 67.

In the illustrative arrangement shown the through flow vapor generator advantageously includes a recirculation system designated generally 50 which is superimposed on a portion of the through flow circuit including the portion of the circuit of highest heat absorption.

The recirculating system includes the recirculating conduit 52 connected at its inlet with the through fiow circuit downstream of the rear gas pass tubes 20 and at its outlet with the mixing vessel 54 where the hot recirculated fluid is mixed with the relatively cold fluid coming from the economizer. Disposed within the conduit 52 is a suitable stop and check valve 56. Also forming part of the recirculated system is the recirculating pump 58, disclosed connected into the through flow circuit and having a suitable bypass 60 with valves being provided upstream and downstream of the pump and with a check valve being provided in the bypass. This recirculation system is eflective to provide a predetermined minimum flow through the portion of the through flow circuit upon which it is superimposed, and the system may be organized to automatically provide a flow that is at least equal to the flow prevailing at a predetermined load. For example, it may become etfective to start to recirculate between 70 and percent of full load and as the load is decreased the recirculating system will insure that the flow through this portion of the through flow circuit is adequate for tube protection. For a detailed explanation of the theory of the recirculating system that forms a part of the modified once through flow vapor generator of the invention, reference is made to U.S. Patent 3,135,252, of Willburt W. Schroedter.

The recirculation pump 58 is electrically driven and it is to be understood that while the present invention is particularly adapted and finds maximum benefit in a supercritical once through flow vapor generator with such a recirculation system as previously mentioned the invention is not limited as to sub or supercritical pressure use or to a once through flow vapor generator with such a recirculating system since substantial advantage is found for the invention in a once through flow vapor generator without such a recirculating system.

During the operation of such a power plant as hereinbefore described it may happen that the main load supplied by the turbo generator is lost for a short time. This may occur when lightning strikes the electrical transmission system and the circuit breakers are thus activated to disconnect the load of the power station. This will require what is termed a hot trip-out or at least a trip-back to what is termed house load. In the hot trip-out the turbine valves 88 will be closed and the vapor supplied to the turbine will be terminated. The vapor generator will then be shutdown; the fire is extinguished and valves 62 will be closed with pressure being retained in the generating section 12, with this pressure, for example in the case of a supercritical pressure unit, being 3500 pounds per square inch or somewhat greater. The feed pump 44 will be de-activated. With the vapor generator in this condition and with it still being hot it may be desirable to restart the power plant a short time after a trip-out or tripback. For this it is necessary to establish a through flow through the generator and with the present invention this is accomplished by supplying vapor to the turbine drive 67 of the feed pump 44. In order that there will be a supply of vapor available for this turbine drive the construction and operation of the vapor generator is such that during normal operation a vapor head is maintained in the separator or flash tank 64 at a pressure much lower than that of the through flow circuit. The tank 64 is sized as a vapor accumulator with it being maintained and operated as such. There is thus a liquid level maintained Within the accumulator, and during normal operation the volume above this level is filled with the vapor while the liquid is maintained at saturation temperature. The size of the tank 64 or in other words the accumulator is such that the volume of vapor obtainable therefrom will be sufiicient to supply the requirements of turbine 67 necessary for the hot re-start operation. It will be appreciated that during the hot re-start operation and incident to withdrawing vapor from the accumulator 64 to drive the turbine 67 the pressure will be reduced somewhat in the accumulator such that at least aportion of the liquid therein will flash to vapor thereby producing additional vapor for operation of this turbine drive.

The vapor head is retained within the tank 64 by continuously heating the fluid in this tank during normal operation of the unit. This heating may be obtained by withdrawing fluid from the through flow circuit through the conduit 66 and passing this fluid in heat exchange relation with the contents of the tank 64. This fluid may be bled through the valve 68 and introduced directly into the tank 64 in direct heat exchange relation with the fluid in the tank. The SP valve will be set to regulate the pressure within the tank 64 with this valve exhausting to condenser 30. In lieu of introducing the fluid from conduit 66 into direct heat exchange with the fluid in the tank 64, an indirect heat exchange may be provided with this fluid passing through a heat exchanger disposed within this tank. In this latter case the fluid supply to the tank 64 may be from any desired source and the supply may be controlled by means of a liquid level control device within the tank. The vapor head thus produced in tank 64 will be such as to insure that a suflicient quantity of vapor is at all times present in this tank to effect a hot restart.

In lieu of obtaining the necessary vapor head within the separator or flash tank 64 by bleeding vapor through conduit 66 this head may be obtained by conveying vapor from the exhaust of the high pressure turbine or other suitable turbine extraction point to the flash tank either in a direct heat exchange arrangement or in an indirect heat exchange arrangement. This is particularly well adapted for a unit operating on the reheat cycle and such an arrangement is disclosed in FIG. 2 wherein there is provided a reheater 90 which receives exhaust vapor from high pressure turbine 26 through the line 92. A portion of this exhaust vapor is conveyed through line 94 and introduced into the separator or flash tank 64. The SP valve will be set to insure that the pressure within tank 64 is retained at its desired value with this valve exhausting to the condenser 30. While FIG. 2 discloses a direct heat exchange arrangement wherein the vapor that is exhausted from high pressure turbine 26 is conveyed directly into the tank 64, an indirect heat exchange arrangement may be provided wherein this vapor flows to a heat exchanger disposed within the tank 64 with the vapor then being returned to the through flow system at a suitable point.

Another way of providing the necessary vapor head within the separator or flash tank 64 is to position within this tank an electric heater which heater is supplied by energy from the generators driven by the vapor produced from the once through flow vapor generator. FIG. 3 illusstrates such an arrangement with the electric heater being designated 98 and being connected to obtain its energy from the generator 93 of the turbo generator unit 95.

It will be noted that each of these several arrangements for heating the fluid within the flash tank 64 during normal operation of the vapor generator eifectively receives its energy from the burning of fuel within the furnace of the vapor generator as through the burner 86. Still another arrangement would be to provide a separate energy source for the flash tank 64 to heat the fluid therein during normal operation of the high pressure high temperature vapor generator. For example, this flash tank could be part of aseparately fired heater. In any event, however, it is to be understood that it is the flash tank which is utilized in the arrangement of the present invention to act as an accumulator for use in effecting a hot re-start.

In the event that there is a trip-out such that the vapor generator is shutdown as previously mentioned and it is necessary to provide a hot re-start of the generator this hot re-start is accomplished by initially activating the feed pump 44 by supplying vapor through line 69 from separator 64. The head of vapor within the tank 64 is such that a limited flow through the section 12 of the through flow vapor generator is obtained. Valves 62, which were closed at the trip-out remain closed. If the unit is provided with a recirculating system the recirculating system is activated to recirculate fluid as previously mentioned and the fires are lit. The through flow is conveyed through the valve 68 which is effective to substantially reduce the pressure and since the fluid is already hot and is being further heated by the introduction of fuel into the furnace through the burners 86 a portion of this fluid will flash into vapor to provide further vapor Within the separator or flash tank 64. As a result of heat being imparted to the through flow circuit the temperature of the fluid at the outlet of the section 12 of the generator is progressively raised. The pressure of this fluid in this generator section is maintained, for example in a supercritical unit, at the aforementioned relatively high value of 3500 pounds per square inch with the throttle valve 68 effectively maintaining this pressure. During the hot re-start operation it may be desirable and from a turbine standpoint will be desirable to de-pressurize the superheater portion In of the through flow circuit. The reason for this is to lessen the chance of termal shock to the turbine during the hot re-start operation. If this depressurization is resorted to, the vapor may be conveyed from this superheater portion to the turbine drive 67 with line '73 being provided in the illustrative arrangement for this purpose. Should it be desired for some reason not to convey this vapor to the turbine 67 or not to provide the line 73 then the de-pressurization may be efiected by manipulating the SD valve to convey vapor from the superheater to the condenser. If the superheater 19 is de-pressurized then in addition to conveying vapor from the tank 64 to the turbine 67 through conduit 6 it may be conveyed through the valve 72 to the superheater 10 and then supplied to turbine 26 for initial activation of the turbine.

When the temperature of the vapor has reached a desired value in the portion 12 of the through flow circuit it may be conveyed from this portion through the valves 62 to the superheater It). Vapor may be continued to be bled to the tank 64 so long as it is required to supply vapor to the turbine drive 67 from this tank. During this startup operation the amount of vapor conveyed to superheater 10 will progressively increase and accordingly the vapor supplied to the high pressure turbine 26 will progressively increase. When the value is sufiiciently high that extraction vapor may be obtained from line 86 in suflicient quantity and pressure for supplying the requirements of the turbine drive 67 the supply of vapor from the tank 64 may be cut off with there being suitable valve means in the line 69 for this purpose.

It will be evident that the system and process of the invention finds its greatest advantage in an arrangement wherein a recirculating system is provided as a portion of the once through flow vapor generator. This is so since the amount of vapor initially necessary to be supplied to the turbine drive 67 will be less than in a once through flow vapor generator without such a recirculating system. The recirculating system will effectively provide a velocity through the high heat absorption portion of the vapor generator so as to insure against overheating this portion. There is thus less through flow required during initial operation. Without the recirculating system the through flow must be adequate to effect this protection and the feed pump 44 must provide this through flow. Since the amount of vapor that can be made available by means of storing vapor Within the tank 64 is necessarily limited the decreased requirement with the system having a recirculating arrangement associated therewith is of particular significance and advantage.

In lieu of completely shutting down the unit during a trip-out of the main load on the turbo generator it may be desirable and in some stations is considered of advantage to maintain the entire power plant in operation but at a very much reduced load such that the electric output is only sufficient to provide the requirement of the power plant with there being in effect no electric power sent out from the power plant. This load may be only in the order of five percent of the maximum power plant output. In the case of such a trip-back the turbine valves 88 will initially be closed with electrical generation at the reduced rate being accomplished from steam bled from the reheater 90 for a short time. Thereafter the valves 88 will be opened to permit a relatively small vapor flow. The firing of the vapor generator will also be decreased consistent with heat input limitations necessary to generate the reduced electrical load. The vapor available at the extraction point to which the line 86 is connected is now insufiicient to drive the turbine 67. Thus the turbine 67 will be driven from vapor contained Within the separator or flash tank 64 to the line 69. Vapor will continuously be produced within this container as by bleeding through the valve 68 or by heating the fluid in the container through the arrangements disclosed in FIG. 2 or 3. In the event that the vapor generator organization is provided with a line such as 73 in FIG. 1 whereby vapor can be conveyed from the outlet of the superheater to the turbine drive of the main feed pump, in addition to conveying vapor from the flash tank 64 during this low load operation vapor may also be conveyed through the line 73 with these two supplies of vapor together being effective to operate the turbine 67. It will be understood that the vapor generator may not have the line 73 since in order to be provided with such a line the turbine 67 must have a high pressure nozzle block, and the economics may be such that this equipment is not justified. In such case the entire vapor supply for the turbine 67 for this low load operation will be obtained from the flash tank 64. The firing of the vapor generator will be continued at a very low rate and this state of opeartion will be continued until the load is again ready to be picked up. At this time the fires will gradually be increased and the supply to the high pressure turbine 26 increased until the vapor available through line 86 is sufficient to drive the turbine 67. At this point vapor will no longer be supplied through the line 69 from the separator or flash tank 64.

While I have illustrated and described a preferred embodiment of my invention it is to be understood that such is merely illustrative and not restrictive and that variations and modifications may be made therein without departing from the spirit and scope of the invention. I therefore do not wish to be limited to the precise details set forth but desire to avail myself of such changes as fall within the purview of my invention.

What I claim is:

1. In the operation of an electric generating power plant wherein there is provided a vapor generator which supplies the motive fluid to a turbo generator unit with said vapor generator being of the once through flow type and having a through flow circuit that includes first and second heating portions connected in series, a valve means interposed between these portions, a flask tank connected in bypass relation with said valve means, valves in the connections of said flash tank, a feed pump and a turbine drive for said feed pump the improved method of operation comprising heating the fluid in the flash tank during normal operation of the vapor generator by means of electric energy produced by said turbo generator unit during normal operation of the power plant, effecting a hot re-start of the vapor generator after a hot trip by initiating activation of the turbine drive for the feed pump by supplying vapor therethrough from said flash tank, thereby creating a sufficient through flow for startup of the vapor generator.

2. In a once through flow vapor generator providing the motive fluid for a turbine generator unit the improved method comprising burning fuel and producing high temperature high pressure vapor, utilizing this vapor in said unit to generate electricity, utilizing a portion of the energy evolved by the burning of the fuel to continuously maintain a source of vapor at a pressure substantially reduced with relation to said high temperature pressure vapor and in sufficient quantity to effect a hot re-start of the vapor generator, incident to a hot trip of the vapor generator effecting a hot re-start thereof by recirculating fluid through the portion of the through flow circuit that has the highest rate of heat absorption during startup, conveying vapor from said low pressure source to a turbine drive to drive the main feed pump to establish a through flow through the through flow circuit of the vapor generator, simultaneously therewith introducing fuel into and burning the same within the vapor generator and raising the temperature of the fluid therein to the desired value for operation of the turbo generator unit.

3. The method of claim 2 wherein said source of low pressure vapor is maintained by bleeding fluid from the through flow circuit.

4. The method of claim 2 wherein the vapor generator is operated on the reheat cycle with the high pressure high temperature vapor being conveyed to a high pressure turbine, a reheater forming part of said vapor generator and receiving the exhaust from the high pressure turbine and a lower pressure turbine receiving the vapor after it has been conveyed through said reheater and wherein said low pressure vapor is produced by conveying vapor after it has been exhausted from the high pressure turbine in heat exchange relation with fluid at said location and prior to the vapor being introduced into the reheater.

5. In the operation of a power plant wherein a once through flow vapor generator supplies high pressure high temperature vapor to a turbo generator unit with fuel being burned in the vapor generator to produce said vapor, said vapor generator having a feed pump driven by a turbine, this turbine receiving its vapor supply from an ex traction point in the turbine of the turbo generator during normal operation of the power plant, the improved method comprising utilizing a portion of the energy evolved by the burning of said fuel to continuously provide a source of vapor at a substantially lower pressure than that of the high temperature high pressure vapor during normal operation of the vapor generator, incident to loss of the main load on the power plant substantially reducing the supply of vapor to the turbo generator unit such that the electrical output is reduced substantially to that required for operation of the power plant, continuing to force fluid through the through flow circuit of the vapor generator during this low load operation by supplying vapor to this turbine drive for the feed pump from said relatively low pressure source, continuing to provide vapor at this low pressure source throughout the period of the reduced operation and increasing the output of the power plant when desired by increasing the firing rate of the vapor generator and the supply of vapor to the turbo generator unit, transferring the supply of vapor to the turbine drive for the feed pump from the low pressure source to said extraction point when adequate vapor is available at the extraction point of the turbine of turbo generator.

6. A forced through flow vapor generator comprising a through flow circuit having first and second heat exchange portions in series flow relation with valve means interposed intermediate these portions, a recirculating system superimposed on at least the region of the first heat exchange portion having the highest rate of heat absorption, a main feed pump, a turbine drive for said pump, a flash tank communicating with the through flow system upstream of said valve and downstream of said first heating section and also downstream of said valve and upstream of said second heat exchange section, throttle valve means operative to control flow from the through flow circuit to said flash tank, a turbo generator effectively connected with and driven by vapor produced in said vapor generator, an electric heater in said flash tank supplied with electric energy from said turbo generator and operative to maintain a vapor head in said flash tank during normal operation of said unit whereby said vapor head will be available for rapid hot re-start of the vapor generator, and means for controllably conveying vapor from said flash tank to the turbine drive for said feed pump.

7. In a once through flow vapor generator providing the motive fluid for a turbine generator unit the improved method comprising burning fuel and producing high temperature high pressure vapor, utilizing this vapor in said unit to generate electricity, heating fluid at a source by electrical energy generated by the turbo generator to maintain a source of vapor at a pressure substantially reduced with relation to said high temperature high pressure vapor and in sufficient quantity to effect a hot re-start of the vapor generator, incident to a hot trip of the vapor gen- 10 erator effecting a hot re-start thereof by recirculating fluid References Cited through the portion of the through flow circuit that has UNITED STATES PATENTS the highest rate of heat absorption during startup, conveying vapor from said low pressure source to a turbine drive g g 122 1 to drive the main feed pump to establish a through flow 5 3194219 7/1965 c 5: et a1 122-406 through the through flow circuit of the vapor generator, Hanza e 122406 simultaneously therewith introducing fuel into .the burn- FOREIGN PATENTS ing the same within the vapor generator and raising the 374,800 6/1932 Great B i i temperature of the fluid therein to the desired value for operation of the turbg generator unit 10 I. NIYHRE, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,356,074 December S, 1967 Frederick J. Hanzalek It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 7, line 63, after "temperature" insert high column 9, line 7, for "the" read and Signed and sealed this 1st day of April 1969.

(SEAL) Attcst:

EDWARD J. BRENNER Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer 

2. IN A ONCE THROUGH FLOW VAPOR GENERATOR PROVIDING THE MOTIVE FLUID FOR A TURBINE GENERATOR UNIT THE IMPROVED METHOD COMPRISING BURNING FUEL AND PRODUCING HIGH TEMPERATURE HIGH PRESSURE VAPOR, UTILIZING THIS VAPOR IN SAID UNIT TO GENERATE ELECTRICITY, UTILIZING A PORTION OF THE ENERGY EVOLVED BY THE BURNING OF THE FUEL TO CONTINUOUSLY MAINTAIN A SOURCE OF VAPOR AT A PRESSURE SUBSTANTIALLY REDUCED WITH RELATION OF SAID HIGH TEMPREATURE PRESSURE VAPOR AND IN SUFFICIENT QUANTITY TO EFFECT A HOT RE-START OF THE VAPOR GENERATOR, INCIDENT TO A HOT TRIP OF THE VAPOR GENERATOR EFFECTING A HOT RE-START THEREOF BY RECIRCULATING FLUID THROUGH THE PORTION OF THE THROUGH FLOW CIRCUIT THAT HAS THE HIGHEST RATE OF HEAT ABSORPTION DURING STARTUP, CONVEYING VAPOR FROM SAID LOW PRESSURE SOURCE TO A TURBINE DRIVE TO DRIVE THE MAIN FEED PUMP TO ESTABLISH A THROUGH FLOW THROUGH THE THROUGH FLOW CIRCUIT OF THE VAPOR GENERATOR, SIMULTANEOUSLY THEREWITH INTRODUCING FUEL INTO AND BURNING THE SAME WITHIN THE VAPOR GENERATOR AND RAISING THE TEMPERATURE OF THE FLUID THEREIN TO THE DESIRED VALUE FOR OPERATION OF THE TURBO GENERATOR UNIT. 